Diabetic nephropathy is characterized by albuminuria/proteinuria, and over-reabsorption of protein/albumin triggers tubulointerstitial inflammation and fibrosis. The goal of the proposed studies is to uncover the molecular mechanism(s) involved in albumin overload-related renal injury in progressive diabetic nephropathy. Tubular Kidney injury molecule-1 (KIM-1) induction was associated with increased MMP-9, myofibroblast activation and albuminuria/proteinuria in diabetic rats. The pilot study revealed that FITC-albumin was preferentially accumulated in KIM-1-expressing tubular cells. Therefore, the central hypothesis is that Tubular KIM-1 expression exacerbates diabetic renal injury by enhancing protein overload-induced tubulointerstitial inflammation and fibrosis. Aim I will test the hypothesis that tubule-specific overexpression of KIM-1 exacerbates diabetic renal injury. Systematic studies will be conducted 1) to characterize the spatial, temporal, and quantitative expression of human KIM-1 in doxycycline-inducible Pax8- rtTA/hKIM1 transgenic mice and 2) to determine if tubule-specific expression of hKIM-1 promotes renal injury in high-fat diet and streptozotocin-induced diabetic mice. Aim II will test the hypothesis that KIM-1 expression enhances protein overload-induced proinflamatory and profibrotic responses in renal tubular epithelial cells. Proposed studies aim 1) to define the correlations among tubular KIM-1 expression, protein reabsorption, and tubular cells activation in the kidneys of Pax8-rtTA/hKIM1 transgenic mice and 2) to examine the effects of KIM-1 expression on tubular proinflammatory and profibrotic responses to albumin, high glucose, and TGF-?1 stimulation in cultured tubular epithelial cells. Aim III will test the hypothesis that inhibition of KIM-1 would prevent protein overload and attenuate diabetic tubulointerstitial damage. Both in vitro and in vivo studies are proposed 1) to examine if inhibiting KIM-1 by neutralizing antibodies or shRNA gene silencers would prevent albumin overload-induced tubular cell activation and injury in cultured tubular epithelial cells and 2) to define if inhibiting KIM-1 expression and activity would prevent excess protein reabsorption by proximal tubular cells and attenuate tubulointerstitial injury in the diabetic kidneys.